In the ongoing effort to determine whether sexual orientation is hardwired, University of Chicago scientists have used high-tech imaging to confirm that the hypothalamus--the sex center in the brain--functions differently in gay men than in heterosexual men.

Scientists have searched for ways to determine if sexual preference is a matter of choice or biology. Still, they have failed to develop convincing evidence one way or the other. Genes once touted as prompting homosexuality, for example, have fizzled out and studies of hormonal influences during fetal development are inconclusive.

Since scientific evidence has been lacking for a biological cause of homosexuality, many politicians, religious leaders and others maintain it is a purposeful choice.

As acceptance of homosexuality has increased, however, so has acceptance of the idea that it is not a choice, but some heretofore unseen cues--either psychological or physical or both--that set sexual preference.

Although the new U. of C. findings suggest male sexual response is regulated in large part by genes or neurochemistry, the results are preliminary and need to be replicated in other studies. And there surely are other factors, both biological and social, that influence the sexual response.

"I don't think homosexuality can easily be conceptualized as just one thing--a phenomenon that is due to one particular developmental pathway," said Heino F. L. Meyer-Bahlburg, Columbia University professor of clinical psychology who was not involved in the research. "Like most behavior, homosexuality has multiple pathways. We're at the crude beginning to understand all of this. This [the U. of C. study] is a promising development and a very exciting one."

The U. of C.'s Howard Moltz, professor emeritus in psychology, headed the team showing that sexual orientation in men appears to be connected with brain metabolism. The report was presented Wednesday at the annual meeting of the Society for Neuroscience in New Orleans by Leann Kinnunen, a U. of C. psychology graduate.

Variations in brain activity

Using positron emission tomography (PET) to monitor the neurochemical function of the hypothalamus in eight exclusively heterosexual men and eight exclusively homosexual men, Moltz found a significantly greater level of activity in straight men compared with gays. The hypothalamus is thought to regulate sexual response and behavior, according to animal studies.

"Whether this neurochemical difference is the cause of, or a consequence of, or something that accompanies this kind of heterosexuality and homosexuality is yet to be determined," Moltz said. "But it's the strongest research I know to suggest that it might be hardwired."

How the hypothalamus functions in lesbian and straight females was not studied, but it is likely that they have patterns of activity that are similar to those of males, Moltz said. "I would expect that a neurochemical difference would show up in lesbians as it did in our exclusively homosexual or heterosexual men," said Moltz, who said he plans to conduct similar studies with women.

Meyer-Bahlburg said the importance of the research was in the use of new technology that provided clear-cut differences.

"The results are stunning because these are very extreme groups, and he finds these pervasive differences that are lighting up on his brain scans that involve a large part of the known sexual circuitry that we know from animals," he said.

Dr. Fred Berlin, a Johns Hopkins School of Medicine psychiatrist, said that the U. of C. findings need to be replicated, but even then they may reflect the current functioning of the brain and not pinpoint the cause of homosexuality. "The important point in terms of the cause of homosexuality, or heterosexuality for that matter, is that it isn't due to choice," he said. "None of us as little children sat down and said to ourselves, `When I grow up do I want to be attracted to men or to women?'

"We simply discover in growing up who it is we're attracted to," Berlin said.

Numerous studies in animals show that the hypothalamus mediates sexual arousal and sexual behavior, but its role in human sexuality has been little studied. And although the hypothalamus may play an important role in human sexual response, other parts of the brain, especially the thought-processing neocortex, are likely to also exert considerable influence.

Gay men were selected for brain scans if they said they had never been aroused by a woman and straight men were chosen if they said they had never been aroused by a male.

Jibes with other data

"This report fits in with an increasing body of data suggesting that sexual orientation has a biological basis," said Simon LeVay. In 1991, while at the University of California, San Diego, LeVay found in autopsy studies that the front section of the hypothalamus of heterosexual men was larger than that of heterosexual women and that the size in gay men was also small, nearly the same size as females.

Another team of scientists last year reported that rams that copulated with other rams had a smaller hypothalamus than rams that copulated with ewes.

Moltz said that other studies on living subjects failed to show significant differences in brain function between gays and straights probably because their samples included men whose sexual preferences included both males and females.

Dr. William Gilmer, a Houston neurologist and past president of the Gay and Lesbian Medical Association, said that there is a growing understanding that "gay and straight brains are wired differently. Sexual orientation is no more a choice for a GLBT [gay, lesbian, bisexual, transgender] person than a straight one."

"This study adds more evidence to the common belief of both gay and straight people that their basic sexual orientation is something they were born with," Gilmer said. "Most gay and straight people know what they think is attractive from their earliest sexual feelings, and those basic feelings don't change much through their lives."